1. Field of the Invention
The present invention relates to calibration of the horizontal axis of an oscilloscope, and more particularly to a method and apparatus for calibrating electronic scales for the horizontal axis.
2. Description of the Related Art
Generally, in oscilloscopes, a horizontal axis deflection circuit must be calibrated such that each position on a horizontal axis scale (positions div 0-div 10, see FIG. 4) of the coordinates on the screen of a CRT represents a corresponding time in any selected time base range (or any sweep time).
Such a method of calibrating the horizontal axis has been disclosed in Japanese Patent Application Public-Disclosure No. 59-148877, entitled "Automatic Calibration Apparatus for a Sweep Waveform Generator." This method performs calibration of the horizontal axis by adjusting the slope of a sweep waveform. More specifically, a measurement is made with respect to an elapsed time in which a positive ramp portion of a sweep voltage waveform (saw-tooth wave) generated at the output of a sweep waveform generator passes through two voltage points selected in a waveform voltage range thereof (or two corresponding points on a horizontal axis scale on the screen). Then, the slope of the positive ramp portion of the sweep waveform is adjusted such that the elapsed time coincides with a target time difference in the selected time base range. Other calibration methods similar to the above have also been disclosed in Japanese Patent Application Public-Disclosure No. 63-106568, entitled "Automatic Calibration Apparatus for a Sweep Waveform Generator" and No. 63-286775 entitled "Automatic Calibration Apparatus for a Ramp Waveform Generator."
The above-mentioned prior arts all relate to calibration of a sweep waveform with respect to a scale fixed on the screen or on-screen scale (this type of scale is hereinafter also called the "physical scale" in order to distinguish it from an electronic scale such as a cursor). The physical scale is inconvenient for performing highly accurate measurements. Japanese Patent Application Public-Disclosure No. 59-148876 entitled "Vertical Calibration Method and Apparatus for an Oscilloscope" discloses a technique for calibrating electronic scales or cursors (a pair of cursors consisting of a reference cursor and a delta (.DELTA.) cursor) for the vertical axis, however, this technique is not intended for calibration of electronic scales with respect to the horizontal axis of an oscilloscope.
Also, the above-mentioned sweep waveform calibration performs adjustment of the slope of the sweep waveform on the basis of a sweep waveform generated at the output of a sweep waveform generator. In other words, the output of a horizontal deflection circuit to which the sweep waveform is applied is not used for calibration. Thus, it is possible for the sweep speed of a sweep waveform calibrated by the prior art not to coincide with the physical scale in a used time base range due to variations in the characteristics of a horizontal sweep circuit (for example, variations in temperature, supply voltage, and characteristics caused by aging of electronic components).